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Unbalanced load on dual-rail PS causing system instability?

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December 19, 2009 4:37:04 PM

I have a home brew system that I'm trying to get another
year or two of use out of before I bite the bullet with a
complete replacement:

AMD Barton 2500+ oc to ~2250mhz
Gigabyte GA-7VAX
EVGA 6200 LE AGP video card
Seasonic 330w power supply
plus one disk, cd & dvd burners, a couple of other PCI cards and a bunch of fans.

Anyhow, I recently upgraded to the Barton from a Semperon 2400+.
The system is still quite stable except when I try to run certain 3D graphic
applications (Bioshock, MS Virtual Earth 3D, Plants vs. Zombies, etc.) that
used to run sluggishly, and now run much better - but only for a couple
of minutes maybe - until the system spontaneously reboots or hangs.

My suspicions keep switching from the graphics card (pretty low end,
but also pretty low power in the scheme of things), to the power
supply (should be enough for this setup, and a reasonably good brand,
from what I read), to the mobo (always been a solid performer with
lot's of bios settings to fiddle with, but maybe showing it's age).

I've been reading up on power supplies and have come up with this:

The Seasonic is a "dual 12v rail" power supply with specs that say:
12V1 - 8amps
12V2 - 14amps

I was trying to figure out exactly what in my system was connected to
which rail, which is no easy feat. I managed to find the ATX12V 1.2
spec which, lo and behold, shows an example of these specs as "typical"
of a 300w PS. So far so good.

The GA-7vax and 6200 LE don't use the aux power connector, so ALL
the power for the CPU & graphics cards (and other cards and some
of the fans) comes through the 20pin ATX main power mobo connector
which the ATX 2.0 spec hints gets 12V from the 12V1 rail. Surprisingly,
the disk, cd/dvd drives, and other case fans that use a standard 4-pin
molex power connector ALSO appear to be drawing power from 12V1 rail
per the ATX spec! (It seems ONLY the special 4-pin AUX 12v connector
which is also part of the newer 24-pin power connector draws from the
12V2 rail??)

In any event, it appears my entire system is running off the 12V1 rail
which is limited to 8amps! Is that possible??

As a potential solution, I was thinking of modding the power supply
connectors and replacing the 12v pin of the 20pin main power connector,
with the one from the 4-pin 12v aux connector drawing from 12V2.

Any thoughts on this? Thanks in advance!
a c 144 ) Power supply
December 19, 2009 7:14:35 PM

alfei said:

In any event, it appears my entire system is running off the 12V1 rail
which is limited to 8amps! Is that possible??

No, it is not. The small and medium sized PSU's have a single 12 volt output that is split into two "virtual" rails. If you use an ohmmeter to measure from any yellow wire (12 volt) to any other, you will read a short circuit.

alfei said:

As a potential solution, I was thinking of modding the power supply
connectors and replacing the 12v pin of the 20pin main power connector,
with the one from the 4-pin 12v aux connector drawing from 12V2.

Any thoughts on this? Thanks in advance!

Yes. Don't do it. It's a complete waste of time.

If this is the original PSU, there's a good chance that it has degraded over the years and is no longer putting out its rated power.

If you are planning to upgrade to a modern PC with one video card, get one of these:
Corsair 550VX
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
and move it to your new system when you build it.

Otherwise, get one of these:
Corsair 400CX:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
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December 20, 2009 2:20:46 PM

Quote:
The small and medium sized PSU's have a single 12 volt output that is split into two "virtual" rails. If you use an ohmmeter to measure from any yellow wire (12 volt) to any other, you will read a short circuit.

OK, but there's got to be some significance to the specs for the 2 different rails that Seasonic puts in the sheet that came with the PSU and on the PSU itself, right? I mean if they just put in 8-amp and 14-amp "fuses" (or circuit breakers, etc) on the two different rails, an ohmmeter would show that, but there still would be a practical difference between the two rails that could cause noticeable effects depending on the loading conditions. I don't think they would just make up specs for "dual rails" with absolutely no engineering behind it! Also, other than the time wasted, is there any other downside to trying the wire switching?

The PSU was part of an upgrade I did 4 years ago. How many years should a PSU typically be good for?

In any case, thanks for the recommendations! I'll definitely keep Corsairs in mind when I'm ready to get a new one.
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Related resources
December 20, 2009 7:21:32 PM

Delluser1: Thanks! Yes those specs for the S12-330 are exactly what's on the lable of my Seasonic PSU. I opened my case to take a look. If Seasonic made more than one type of S12-330, I definitely have the dual-rail version.
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December 20, 2009 8:01:25 PM

Oh, and one correction to something I wrote about the ATX 2.0 spec in my original post: The 4-pin extension to the original 20-pin ATX mobo power connector is NOT the same thing as the 4-pin aux connector (or more precisely the "+12V Power Connector"). The extension is +12v, +5v, +3.3v, and COM, while the "+12v Power Connector" is 2 COM's and 2 +12V). But most relevant to this discussion, the second +12v pin of the now 24-pin mobo power connector still appears to supply power only from the first rail, 12V1!
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a c 144 ) Power supply
December 21, 2009 12:06:14 PM

DellUser, he said "old". :shrug:
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a c 243 ) Power supply
December 21, 2009 1:56:46 PM

jsc said:
DellUser, he said "old". :shrug:

Here's a review from 4 years and 8 months ago
http://www.phoronix.com/scan.php?page=article&item=166&...
That's fairly old.
It's gone thru several revision change's but has always been dual rail.
It's not your fault dealtime didn't get the spec's right :) 
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December 21, 2009 6:06:23 PM

Another curious observation. According to these specs on Seasonic's web site:

http://www.seasonicusa.com/s12.htm

as you go up from a 330w to a 600w PSU, one of the main effects is a large increase in amps for the +12V1 rail. So it seems to me, I'd be gettting the rail #1 power equivalent to a S12-430, effectively "upgrading" my power supply from 330w to 430w (for my purposes) just by swaping the wires!
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